1. Field of the Invention
The present invention relates to a hydraulic circuit for heavy equipment which can utilize a part of flow rate of a hydraulic pump that drives a hydraulic pump for a cooling fan, as a hydraulic power source of remote control valve lever, and more particularly, to a hydraulic circuit for heavy equipment which can utilize a hydraulic fluid supplied from a hydraulic pump that drives a cooling fan, as a pilot signal pressure, without installing a constant displacement pilot pump for supplying the pilot signal pressure to a control valve that controls the hydraulic fluid to be supplied to a working device such as a boom.
2. Description of the Prior Art
FIG. 1 shows a conventional hydraulic circuit for heavy equipment comprising first and second variable displacement hydraulic pumps 2 and 3 and third and fourth constant displacement hydraulic pumps 4 and 15 which are connected to an engine 1; a first control valve 5 installed in a flow path of the first variable displacement hydraulic pump 2 and controlling the hydraulic fluid to be supplied to an actuator that drives a working device, such as a boom, a bucket, a traveling device, or the like, by using a pilot signal pressure supplied from the fourth hydraulic pump 15; a second control valve 5a installed in a flow path of the second variable displacement hydraulic pump 3 and controlling a hydraulic fluid to be supplied to an actuator that drives a working drive, such as a swivel device, an arm, a traveling device, or the like, by using a pilot signal pressure supplied from the fourth hydraulic pump 15; a hydraulic motor 9 connected to the third constant displacement hydraulic pump 4; a cooling fan 10, connected to and rotated by the hydraulic motor 9, for discharging cooling wind towards an oil cooler 11 to lower a temperature of the hydraulic fluid drained to a hydraulic tank T through a return flow path 16; a temperature sensor 13 for detecting the temperature of the hydraulic fluid of the hydraulic tank T; an electric relief valve 12, installed in a drain flow path 17 of the third hydraulic pump 4, for controlling the hydraulic pressure that drives the hydraulic motor 9, to variably control rotation velocity of the cooling fan 10; and a controller 14 for varying a set pressure of the electric relief valve 12 in response to a detected signal from the temperature sensor 13 to control the hydraulic pressure that drives the hydraulic motor 9.
In case where the first and second control valves 5 and 5a are switched by the pilot signal pressure supplied from the fourth hydraulic pump 15 in accordance with the switching of a pilot pressure generator 6, the inner spools of the first and second control valves 5 and 5a controlling the hydraulic fluid supplied to the actuator from the first and second hydraulic pumps 2 and 3 will not be shown and described herein.
The pilot pressure generator 6 is connected to the fourth constant displacement hydraulic pump 15, and generates the pilot signal pressure to a driver at the switching. Reference numeral 6 denotes a relief valve installed in the flow path 18 of the fourth hydraulic pump 15 and draining the hydraulic fluid to the hydraulic tank T when a load exceeding the pressure set in the fourth hydraulic pump 15 generates.
As the inner spools of the first and second control valves 5 and 5a are shifted in accordance with the switching of the respective pilot pressure generator, the working device such as a boom is driven by the hydraulic fluid supplied to the actuator from the first hydraulic pump 2, and the swivel device is driven by the hydraulic fluid supplied to the actuator (e.g. a swing motor) from the second hydraulic pump 3.
The hydraulic motor 9 is driven by the hydraulic fluid supplied from the third hydraulic pump 4 along the drain path 17, and as the cooling fan 10 is driven by the hydraulic motor 9, the temperature of the hydraulic fluid passing through the oil cooler 11 installed in a return path 16 and returned to the hydraulic tank T.
The intensity of cooling blast discharged from the cooling fan 10 to the oil cooler 11 is in proportion to the rotation velocity of the cooling fan 10, and as the rotation velocity of the cooling fan 10 is increased, the load pressure of the hydraulic motor 9 is proportionally increased.
In this instance, the load pressure of the hydraulic motor 9 is controlled by the electric relief valve 12. More specifically, if the load pressure of the hydraulic fluid supplied to the hydraulic motor 9 from the third hydraulic pump 4 exceeds the set pressure of the electric relief valve 12, the hydraulic fluid supplied from the third hydraulic pump 4 passes through the electric relief valve 12 and is drained to the hydraulic tank T. Consequently, the rotation velocity of the cooling fan 10 is controlled by the set pressure of the electric relief valve 12.
The temperature of the hydraulic fluid is raised when the working device such as a boom is driven. When the hydraulic fluid returned to the hydraulic tank T from the actuator passes through the oil cooler 11 installed in the return path, the temperature of the hydraulic fluid is lowered by the cool blast discharged from the cooling fan 10.
More specifically, as the detected signal corresponding to the temperature of the hydraulic fluid of the hydraulic tank T which is detected by the temperature sensor 13 is put in the controller 14, the controller 14 varies the set pressure by transmitting the control signal to the electric relief valve 12 so as to maintain the temperature of the hydraulic fluid in a set value.
For example, if the temperature of the hydraulic fluid stored in the hydraulic tank T exceeds the set temperature, the controller increases the set pressure of the electric relief valve 12 to increase the operation pressure which drives the hydraulic motor 9, thereby increasing the rotation velocity of the cooling fan 10 and thus improving the cooling capacity of the oil cooler 11.
With the conventional hydraulic circuit for the heavy equipment shown in FIG. 1, the fourth constant displacement hydraulic pump 15 discharges a constant amount of the hydraulic fluid in accordance with the rotation of the engine 1. The hydraulic fluid discharged from the fourth hydraulic pump 15 is momentarily used as the pilot signal pressure to switch the switch valves 5 and 5a when the pilot pressure generator 6 is switched.
When the load exceeding the set pressure is generated in the pilot flow path 18, the hydraulic fluid discharged from the fourth hydraulic pump 15 is drained to the hydraulic tank T through the relief valve 8, which leads to the power loss.
That is, power loss=(set pressure of relief valve 8)×(amount of hydraulic fluid to be drained to hydraulic tank T).
Since the pilot pump 15 is connected to the engine 1, the construction of the hydraulic circuit becomes complex, and a cost thereof is thus increased.
FIG. 2 shows another conventional hydraulic circuit for the heavy equipment.
The hydraulic circuit includes a hydraulic pump 50, an actuator 51 connected to the hydraulic pump 50, a solenoid valve 52 installed in a flow path 59 between the hydraulic pump 50 and the actuator 51 and controlling start, stop and direction change of the actuator 51, a sequence valve 56 installed in the first flow path 55 connecting a main inlet port 53 with a primary pressure outlet port 54, and a pressure reducing valve 58 installed in a secondary flow path 57 branched from the primary flow path 55 to constantly maintain the pressure of the secondary pressure output port 60.
With the construction of the conventional hydraulic circuit shown in FIG. 2, since the sequence valve 56 is installed in the flow path 59 between the hydraulic pump 50 and the solenoid valve 52, unnecessary power loss is incurred between the hydraulic pump 50 and the solenoid valve 52.